Conventionally, spark plugs are used in an internal combustion engine. Such spark plugs typically have an insulator having a through hole, a center electrode which is disposed at a front end side of the through hole, a metallic terminal which is disposed at a rear end side of the through hole and a connecting portion which connects electrically the center electrode and the metallic terminal in the through hole.
In fabricating a spark plug, a metallic terminal is inserted into a through hole so as to press a material (for example, a material containing glass) for a connecting portion which is disposed within the through hole in an insulator. If an excessive force is transmitted to the insulator through the metallic terminal, there is a possibility that the insulator may break. Additionally, in the event that the material for the connecting portion is not pressed sufficiently, the durability (for example, loaded life properties) of the connecting portion may be reduced.
A main advantage of the invention is to reduce the possibility of breakage of the insulator by suppressing the reduction in durability of the connecting portion.
The invention has been made with a view to solving at least part of the problem described above and can be realized as an application example which will be described below.